1. Field of the Invention
The present invention relates to an optical pickup of a recording/reproducing apparatus for an optical recording medium such as an optical disc, and more particularly, to an optical pickup capable of controlling the optimum focusing position of a light beam focused on a recording plane of a recording medium such as an optical disc having multiple recording layers, and a focusing control method therefor.
2. Description of the Related Art
There has been developed a high recording density and mass storage information recording medium referred to as a DVD (Digital Versatile Disc) and a system using the medium. A DVD having a single recording layer on one side of the disc has a capacity of 4.7 GB. There has been developed a two-layer DVD which is compliant with the DVD standard to increase the recording capacity. The use of a high NA optical system and a shorter wavelength light-source has been studied for the next generation multi-layer optical disc system utilizing an optical disc having three or four recording layers in order to further increase the recording capacity. In such a multi-layer optical disc having multiple recording layers stacked alternately with spacer layers therebetween, the focal point (the focused point or optimum condensing position) of a light beam should be set to a recording surface of a desired layer. In other words, a condensed light spot must be irradiated upon a desired recording layer in order to read information from one side of the optical disc using an optical pickup.
The focus setting is generally performed by focus pull-in operation (hereinafter, also simply referred to as “focus pull-in”) of servo control in response to a focus error signal FE detected by a focus error detection optical system in order to set the focal point to a desired recording layer. In view of performing reproduction from the multi-layer disc, the focus error signal FE is set so that the capture range (i.e., the focus shift or displacement distance corresponding to the distance between maximum amplitude peaks in the S curve of the signal FE) is set to a value sufficiently smaller than the minimum layer interval (i.e., the thickness of the intermediate layer) among the layer intervals, for example to 1/10 of the layer interval or less. This is for the purpose of excluding as much as possible the effect of light reflected from a recording layer to be reproduced upon other recording layers such as a focus offset caused by interlayer crosstalk. The capture range is determined based on the size or properties of optical elements and light receiving elements in a detection optical system. Focus pull-in operation for a three-layer optical disc 1 shown in FIG. 1 will be considered. A first recording layer is formed on the substrate of the optical disc 1, a second recording layer is formed thereon with an intermediate layer therebetween and a third recording layer is formed thereon with an intermediate layer therebetween. The surface of the third recording layer is protected by a cover layer.
FIGS. 2A and 2B show changes in the focus error signal FE when the relation between the layer interval in the optical disc 1 and the capture range is changed. FIG. 2A shows the focus error signal FE when the capture range is 1/10 of the layer interval. Since the focus error signals from the recording layers have little effect upon each other, a light spot can be focused on a desired recording layer in response to the resulting composite focus error signal FE. However, as can be seen from FIG. 2B showing the focus error signal FE when the capture range is ¼ of the layer interval, the focus error signals from the recording layers affect each other, and the resulting composite focus error signal FE (solid line) has an error, and therefore a light spot cannot be focused on a desired recording layer. Note that various detection methods have been proposed for the focus error detection optical system such as astigmatism, Foucault, and spot size methods, while the above-described conditions are essential for any of the detecting methods in recording/reproducing information to/from the multi-layer optical disc. The capture range must be sufficiently small in comparison with the recording layer interval.
However, a reduction in the capture range makes the focus pull-in difficult, and a disturbance such as vibration during recording/reproducing operation easily interrupts the focus servo control. In the worst case, the focus servo operation could be out of control, and the objective lens could collide against the surface of the optical disc and damage it. In other words, the playability is seriously impaired. If the above conditions are to be met with a certain sufficient capture range, the recording layer interval must be increased. This means that the distance between the disc surface and each the recording layers must be increased. Accordingly, spherical aberration increases when a high NA objective lens is used and therefore, the use of the high NA objective lens becomes an impediment for the use of the multi-layer optical disc.
Therefore, the layer intervals must be small in order to reduce the spherical aberration caused by the high NA objective lens in association with changes in the thickness of the cover layer. The capture range must be small since the ratio of the layer interval and the capture range must be large. When NA is 0.85, the tolerable distance between the first layer and the furthermost layer (i.e., the n-th layer of an n-layer optical disc) is about 20 μm or less, and the capture range is 2 μm or less for a two-layer optical disc, 0.7 μm or less for a three-layer optical disc, and 0.5 μm or less for a four-layer optical disc. In other words, the capture range decreases as the number of layers increases.